Un processo all’interfaccia

1. Un processo all’interfaccia • Per: O + ne- = R • 5 eventi separati devono verificarsi: • O devono essere trasportati con successo dal bulk della soluzione (transporto di massa) • O deve adsorbirsi in modo transiente sulla superficie dell’elettrodo (non-faradico) • dveve avvenire un trasferimento di carica tra elettrodo ed O (faradico) • R deve desorbirsi dalla superfiie elettrodica (non-faradico) • R deve essere trasportato lontano dall’elettrodo nel bulk della soluzione (transporto di massa)

2. Classificazione degli Elettrodi • si basa sulla natura e sul numero delle fasi tra cui avviene il trasferimento elettronico • 3 Classi: • Elettrodi di I specie • Elettrodi di II specie • Elettrodi di III specie

3. Elettrodi di I specie • Metallo in contatto con suoi cationi o non-metallo in contatto con suoi anioni • ESEMPI: • Cu2+ /Cu(s) • Zn2+/Zn(s) • SHE • Ag+/Ag (Elettrodo di riferimento non acquoso) • Cl-/Cl2(g)/Pt Electtrodi nella pila Daniell

4. Elettrodidi I specie • Risposta dell’elettrodo data dalla equazione di Nernst (Nernstiano): • N.B.: elettrodi di Fe, Al, e W NON sono elettrodi di I specie • spessa ricopertura superficiale da parte degli ossidi

5. Electrode of the Second Kind • Metal in contact with sparingly soluble salt of the metal • Common name: anion electrodes • EXAMPLES: • Ag/AgCl(s) • Hg/Hg2Cl2(s)/Cl- (saturated calomel electrode; SCE)

6. Ag/AgCl, KCl Il sale insolubile è AgCl sottoposto all’equilibrio di solubilità: La reazione redox corrispondente è: Essendo insolubile è presente come corpo di fondo o deposito sul metallo aAg+=1

7. Il potenziale redox è relativo alla coppia Ag/Ag+:

8. Electrode of the Second Kind • Electrode response given by: • NOTES: • anion activity determines potential • make great reference electrodes because of low solubility of salt (potential very stable)

9. The Calomel Reference Electrode Note: concentrations typically high  concentrations small  electrode doesn’t become polarized  potential constant

10. Electrode of the Third Kind • Electrodes that merely serve as sources or sinks for electrons • Common names: redox, inert, unattackable • EXAMPLES: • metals: Pt, Au, GC, graphite, HOPG, Hg • semiconductors: Si, GaAs, In-SnO2/glass • Response: • for Pt in contact with Fe2+, Fe3+ in solution: • E = E0- 0.059 (V) log ([Fe2+]/[Fe3+])

11. Electrode of the Fourth Kind • Electrodes that cannot be classified as 1-3 • EXAMPLES: • Chemically modified electrodes (CME’s)

12. Reference Electrodes • Purpose: provide stable potential against which other potentials can be reliably measured • Criteria: • stable (time, temperature) • reproducible (you, me) • potential shouldn’t be altered by passage of small current = not polarizable • easily constructed • convenient for use

13. Advantages International standard E0 0 V One of most reproducible potentials + 1 mV Disadvantages Convenience Pt black easily poisoned by organics, sulfide, cyanide, etc. Hydrogen explosive Sulfuric and hydrochloric strong acids SHE

14. Aqueous SCE Ag/AgCl Nonaqueous Ag+/Ag pseudoreferences Pt, Ag wires Ferrocene Practical Reference Electrodes

15. Cl-(aq)/Hg2Cl2/Hg(l) Hg22+ + 2e- = 2Hg(l) E0 = 0.24 V vs. SHE @ 250C Disadvantages Hg toxic solubility of KCl temperature dependent dE/dT = -0.67 mV/K (must quote temperature) SCE • Advantages • Most polarographic data ref’d to SCE From BAS www-site: http://www.bioanalytical.com/

16. Ag wire coated with AgCl(s), immersed in NaCl or KCl solution Ag+ + e- = Ag(s) E0 = 0.22 V vs. SHE @ 250C Disadvantages solubility of KCl/NaCl temperature dependent dE/dT = -0.73 mV/K (must quote temperature) Ag/AgCl • Advantages • chemical processing industry has standardized on this electrode • convenient • rugged/durable From BAS www-site: http://www.bioanalytical.com/

17. Ag+ + e-= Ag(s) requires use of internal potential standard Disadvantages Potential depends on solvent electrolyte (LiCl, TBAClO4, TBAPF6, TBABF4 Care must be taken to minimize junction potentials Ag+/Ag • Advantages • Most widely used • Easily prepared • Works well in all aprotic solvents: • THF, CAN, DMSO, DMF From BAS www-site: http://www.bioanalytical.com/

18. Pseudo-References • Pt or Ag wire (inert) • Idea:in medium of high resistance, low conductivity, wire will assume reasonably steady, highly reproducible potential (+20 mV) • Advantage: no solution contamination • Limitation: must use internal potential standard (ferrocene)

19. Can Aqueous References Be Used in Nonaqueous Media? • Yes with caution! • May be significant junction potentials • Requires use of internal standard • May be greater noise • Electrolyte may precipitate/clog electrode frit • Don’t forget about your chemistry • Chemistry may be water sensitive

20. Electrodes • Metal • solid • Pt, Au, Ag, C • liquid • dropping mercury electrode (DME) • Semiconductors • Si, GaAs • In-SnO2/glass (optically transparent)

21. Carbon • Paste • With nujol (mineral oil) • Glassy carbon (GC) • Amorphous • Pyrolytic graphite - more ordered than GC • Basal Plane • Edge Plane (more conductive)

22. Electrode Materials • Different Potential Windows • Can affect electron transfer kinetics

23. Electrodes • Size • Analytical macro • 1.6 - 3 mm diameter • Micro • 10-100 m diameter From BAS www-site: http://www.bioanalytical.com/

24. Electrode Geometry Geometry is critical and affects how the data are analyzed and interpreted • Disk • area: r2 • wire (cylinder) • area: l(2 r) r2 • Mesh • optically transparent • Sheet Note: Geometric area < effective surface area

25. Cleanliness IS Next to Godliness in Electrochemistry • Working electrode must be carefully cleaned before each experiment • Mechanical • Abrasion with alumina or “diamond” polish • Chemical • Sonicate in Alconox • Soak in HNO3 • Electrochemical • Cycle in 0.5 M H2SO4 (Pt)

26. Electrochemical Cleaning Taken from Table 4-7 in Sawyer, D.T.; Roberts, Jr., J.L. Experimental Electrochemistry for Chemists Wiley: New York, 1976.